This invention relates to self-converging color picture tube or kinescope display systems that do not require precise transverse, or tilt, alignment between the deflection yoke and the electron beams of the kinescope.
Color television kinescopes or picture tubes create color images by causing electrons to impinge upon phosphors having different-wavelength emissions. Normally, phosphors having red, green and blue-light emission are used, grouped into trios or triads of phosphor areas, with each triad containing one phosphor area of each of the three colors.
In the kinescope, the phosphors of each of the three colors are excited by an electron beam which is intended to impinge upon phosphors emitting only one color. Thus, each electron beam may be identified by the color emitted by the phosphor which the beam is intended to excite. The area impinged on by each electron beam is relatively large compared with a phosphor triad, and at any position on the screen, each beam excites a particular color phosphor in each of several triads. The three electron beams are generated by three electron guns located in a neck portion of the kinescope opposite the viewing screen formed by the phosphors. The electron guns are oriented so that the undeflected beams leave the gun assembly in converging paths directed towards the viewing screen. For the viewing screen to display a faithful color reproduction of a scene it is necessary that the beam position relative to the kinescope be adjusted for producing color purity and static beam convergence at the center of the screen. The purity adjustment involves causing each of the red, green and blue beams to excite only its respective phosphor. This is accomplished by the shadow mask. The shadow mask is a screen or grill having large numbers of perforations through which the electron beams may pass. Each perforation is in a fixed position relative to each triad of color phosphor areas. The electron beams pass through one or more of the perforations and fall upon the appropriate color phosphors based upon their directions of incidence. Color purity depends upon a high order of accuracy in the placement of the phosphor triads relative to the perforations and the apparent source of the electron beams.
Static convergence involves causing the three beams to converge at one scanning spot at or near the center of the viewing screen. Convergence at the center of the screen may be accomplished by the use of a static convergence assembly mounted relative to the neck of the kinescope and adjusted or magnetized to produce a static magnetic field which causes the three beams to converge at the center of the viewing screen.
In order to form a two-dimensional image, the luminescent spot excited on the viewing screen by the three converged electron beams must be scanned both horizontally and vertically over the viewing screen to form a luminescent raster area. This is accomplished by means of magnetic fields produced by a deflection yoke mounted upon the neck of the kinescope. The deflection yoke deflects the electron beams with substantially independent horizontal and vertical deflection systems. Horizontal deflection of the electron beams is provided by coils of the yoke which produce a magnetic field having mainly vertically-directed field lines. The magnetic field intensity is varied with time at a relatively high rate. Vertical deflection of the electron beams is accomplished by coils producing mainly a horizontally-directed magnetic field which varies with time at a relatively low rate. A permeable magnetic core is associated with the yoke coils. The conductors of the coils may enclose the core to form a toroidal deflection winding, or the conductors may form saddle coils which do not enclose the core.
The kinescope viewing screen is relatively flat. The electrons of each electron beam will traverse a greater distance when deflected towards the edge of the viewing screen than when directed toward the center. Due to the separation of the electron guns, this may result in a separation of the landing points of the three electron beams when near or deflected towards the edge of the screen. In addition, prior art almost-uniform magnetic deflection fields caused the electron beams to be overconverged when deflected away from the center of the screen. These effects combine to cause the light spots of the three beams at points on the viewing screen away from the center to be separated. This is known as misconvergence and results in color fringes about the edges of the displayed images. A certain amount of misconvergence is tolerable, but complete separation of the three illuminated spots is generally not acceptable. Misconvergence may be measured as a separation of the ideally superimposed red, green and blue lines of a crosshatch pattern of lines appearing on the screen when an appropriate test signal is applied to the picture tube. Each of the three electron beams scans a raster, which may be identified by its color. Thus, a green raster is ordinarily scanned by the center electron beam, and the outside beams scan red and blue rasters. The crosshatch pattern is formed in each of the red, green and blue rasters. The crosshatch pattern outlines the raster with vertical and horizontal lines, and also includes other vertically and horizontally-directed lines, some of which pass through the center of the raster.
Formerly, kinescopes had the electron guns in a triangular or delta configuration. Convergence of the electron beams at points away from the center of the viewing screen was accomplished in delta-gun systems by dynamic convergence arrangements including additional convergence coils mounted about the neck of the kinescope and driven at the deflection rates by dynamic convergence circuits to excite pole pieces located within the neck of the kinescope to thereby impart corrective motion to the beams, as described in U.S. Pat. No. 3,942,067 issued Mar. 2, 1976 to Cawood.
As described in U.S. Pat. No. 3,789,258 issued Jan. 29, 1974 to Barbin, and in U.S. Pat. No. 3,800,176 issued Mar. 26, 1974 to Gross, et al., current television display arrangements utilize an in-line electron gun assembly together with a deflection yoke arrangement including deflection windings for producing negative horizontal isotropic astigmatism and positive vertical isotropic astigmatism such that the beams are substantially converged at all points on the raster. This eliminates the need for dynamic convergence apparatus in the colro TV display system. However, the nonuniform magnetic fields providing the isotropic astigmatisms necessary for self-convergence make the convergence dependent upon the position of the longitudinal axis of the yoke relative to the longitudinal axis of the undeflected beams. This sensitivity and the normal manufacturing tolerances affecting beam position in the tube, make it necessary to adjust the yoke transversely to achieve the best compromise convergence. A description of the magnitude of the convergence change resulting from a change of the position of the beams relative to the yoke axis appears in the aforementioned Barbin patent.
In order to provide clearance to allow the deflection yoke to be moved transversely (or tilted, which is accomplished by a transverse positioning of the free end of the yoke) relative to the electron beams in order to provide the best overall convergence over the surface of the screen, the diameter of the inner contour of prior-art deflection yokes is made larger than that of the corresponding contour of the envelope of the kinescope by a small amount, such as between 2 and 6 mm.
It is desirable to reduce, insofar as possible, the amount of materials used in the construction of deflection yokes. In order to accomplish this, the deflection yoke should be designed to closely hug the neck portion of the kinescope. Due to manufacturing tolerances, the inner design contour of the deflection yoke must be larger than the nominal outer contour of the kinescope neck, such that the worst-case smallest inner diameter of the yoke will fit snugly over the worst-case maximum outer diameter of the neck. In such a design, the deflection yoke is considered as fitting substantially snugly over the neck of a kinescope even though a gap may occur between the average inner diameter of the yoke and the average diameter of the neck.
Such a snug-fitting yoke will have substantially all of the magnetic flux generated by the coils within the neck of the kinescope. A deflection yoke which does not fit snugly, on the other hand, has magnetic flux in the interstice between the yoke and the kinescope neck. Flux outside of the neck is not used for deflection and merely adds to the total energy which must be stored in the yoke field in order to accomplish a given amount of deflection. Since the stored energy must be periodically added to and removed from the deflection yoke, increased reactive scanning power is required and yoke losses correspondingly increase for yokes which do not fit snugly about the neck of the kinescope. A deflection yoke which fits snugly about the neck of the kinescope may, therefore, be driven by deflection circuits which supply less reactive power, and will dissipate less yoke power. The resulting display system may be expected to have higher deflection sensitivity and be more reliable than displays with loose-fitting yokes. The position-sensitivity of the self-converging deflection windings heretofore used required that the deflection yoke be adjusted by a transverse motion as described in order to accomplish the desired convergence and consequently it has not been possible to provide a mass-produced self-converging yoke fitting snugly about the neck of a kinescope.
Prior art convergence adjustments by positioning the self-converging yoke relative to the beams have been made in various ways. As described in the aforementioned Barbin patent, a kinescope may first be fitted with the deflection yoke with which it is to be used. Static convergence adjustments are then made, the yoke is then moved transversely in vertical and/or horizontal directions to achieve best possible convergence and is then fixed in position by means such as glue or a suitable fastening arrangement. Such a yoke may at the time of its manufacture have been tested in conjunction with a standard kinescope to verify that its characteristics fall within a certain tolerance, i.e., that it is not defective. In a color TV display system currently produced by a major manufacturer, the Barbin technique is used in a two-step fashion. In this system, the picture tube incorporates features by which it can be individually adjusted with a standard deflection yoke during the last stage of manufacture, and yoke locating means are set in position on the tube based upon this adjustment. This system also uses a pre-aligned deflection yoke having mating locating means. In addition, an adjustable circuit associated with the yoke permits electrical compensation for the effects of any remaining horizontal misalignment of the beams in the vertical deflection field. Since every tube and every deflection unit is thus individually pre-aligned, any tube automatically matches with any deflection unit and presumably, the deflection unit only has to be pushed onto the neck of the tube until it seats and requires no further adjustment by the ultimate user.
It is desirable to eliminate this costly pre-alignment of each individual tube for the standard yoke. It is also desirable to provide a self-converging in-line gun television display system which achieves substantial convergence of three beams over the whole raster without the need for transverse or tilt adjustment of the yoke relative to the undeflected electron beams in the kinescope. A self-converging deflection yoke according to an embodiment of the invention not only requires no transverse alignment or pre-alignment for best convergence, but is incapable of being aligned for self-convergence because motion of the yoke relative to the kinescope does not substantially affect convergence. Heretofore, this result has been regarded as self-contradictory, for it was believed that the non-uniform deflection fields necessary to achieve self-convergence by differential deflection of the electron beams made the convergence dependent upon precise alignment of the yoke field with the longitudinal axis of the undeflected electron beams. For example, U.S. Pat. No. 4,060,836 issued Nov. 29, 1977 to Corbeij, et al., states that coincidence of the axes of the deflection field and the display tube is a condition to achieving convergence without additional aids. As a result of the lack of convergence sensitivity, the self-converging yoke in one embodiment of the invention may fit snugly about the neck of the kinescope.
Incremental sensitivity of convergence to vertical and horizontal motion of the yoke about its centered position relative to the beams can be measured to yield a dimensionless ratio of convergence error of the outer beams divided by yoke motion. Ordinarily, convergence error is measured in millimeters, so the ratio represents mm error/mm yoke motion. Yoke motion in a single plane may result in a convergence error at the ends of both directions of deflection. For example, horizontal motion of the yoke from that positon which yields best convergence may cause a change or error in the width of the red raster relative to the blue as well as a relative change or error in height of these two rasters. In particular, horizontal displacement of the beams in the yoke field causes the raster scanned by the leading beam, i.e., the beam which is offset in the direction of displacement, to scan a raster which is increased in width and height relative to that scanned by the lagging beam. Similarly, a vertical motion of the yoke relative to the kinescope may cause an apparent relative rotation or crossover of the central horizontal as well as vertical crosshatch lines displayed on the raster. In particular, a displacement of the beams upward in the yoke field causes the central crosshatch lines scanned by the right-hand offset beam (as viewed from the screen or exit end of the yoke) to rotate clockwise, and those scanned by the left-hand beam to rotate counterclockwise. Vertical movement downward reverses the directions of rotation. Measurements have been made of the incremental sensitivity of convergence to motion of a number of recent display systems including deflection yokes. The results in mm/mm are summarized and tabulated as follows:
______________________________________ HORIZ. MOTION VERT. MOTION Width Height Horiz. Vert. System Error Error Crossover Crossover ______________________________________ Hitachi 0.2 0.8 0.5 0.7 17V 90.degree. semitoroidal Philips 20AX 0.5 0.3 0.5 0.3 25V 110.degree. saddle Philips 30AX 0.9 1.0 0.6 0.1 25V 110.degree. saddle Some RCA systems not embodying the present invention gave the following results: XD4780 19V 90.degree. 1.7 0.8 1.2 0.6 full toroidal XD5000 13V 90.degree. 0.6 0.7 0.5 0.5 semitoroidal XP74-125Q 2.8 1.2 1.6 0.3 25V 110.degree. full toroidal ______________________________________
The 20AX and XP74-125Q systems exhibit relatively low vertical crossover errors because both the 20AX and XP74-125Q display systems are not completely self-converging but use dynamic convergence for top-bottom convergence. The reduced height error in response to horizontal motion of the yoke and reduced vertical crossover error in response to vertical motion result from the reduced vertical astigmatism made possible by the use of dynamic convergence in the 20AX display system.
A mathematical description of the dimensioning of a self-converging yoke is provided by third-order aberration theory as follows. Third-order aberration theory of magnetic deflection can be used to analyze the approximate electron-optical performance of a yoke from its field distribution functions H.sub.0 (z) and H.sub.2 (z) which vary with the position along the longitudinal or z axis of the yoke as is described in two articles entitled "Errors of Magnetic Deflection" by J. Haantjes and G. J. Lubben (H&L) which respectively appeared in Philips Research Reports, Vol. 12, pp. 46-48, 1957 and in Vol. 14, pp. 65-97, 1959. The system of notation adopted herein follows that of H&L.
The deflection of the electron beams taking into account only H.sub.0 (z), the main component of the deflecting field, is termed Gaussian and is designated X or Y. A more complete representation of the field includes H.sub.2 (z), which represents the transverse nonuniformity of the yoke field.
While the description of a yoke field by the field distribution functions H.sub.0 (z) and H.sub.2 (z) is not rigorously applicable to total deflection angles greater than 75.degree., the trends indicated by this field description are useful in outlining performance of mangetic deflection systems with wider total deflection angles, such as 90.degree. and 110.degree..
The deflection fields are described by a power-series expansion about the electron-optical axis of the yoke in such a manner that in the horizontal plane (y=0) the horizontal deflection field is: EQU H.sub.IIy =H.sub.II0 (z)+H.sub.II2 (z)x.sup.2 +. . . (1)
where the yoke axis lies along the z-axis of the coordinate system, and the vertical deflection field in the vertical plane (x=0) is: EQU H.sub.Ix =H.sub.I0 (z)+H.sub.I2 (z)y.sup.2 +. . . (2)
The subscript I refers to a magnetic field having its main component in the x-direction i.e., the vertical deflection field, and subscript II refers to a field having its main component in the y-direction i.e., the horizontal deflection field.
The general aberration expressions describe the differences .DELTA.x and .DELTA.y at the viewing screen between the Gaussian deflection and the third-order deflection (i.e., with H.sub.2 (z) taken into account). These expressions for .DELTA.x and .DELTA.y are simplified in the case of a kinescope with in-line electron beams by eliminating terms relating to entrance of the beams into the yoke field with slopes other than in the horizontal plane.
For in-line electron beams, the aberration expressions pertinent to the invention are: ##EQU1## Here, X.sub.s and Y.sub.s are the Gaussian deflections at the screen, x.sub.s ' is the slope in the horizontal plane of the beam entering the yoke field and x.sub.s, y.sub.s are the coordinates or the landing point of the undeflected beam measured from the trace of the yoke axis on the screen. Equations (3) and (4) are partial, in that only terms relating to the invention, i.e., North-South (NS) pincushion, convergence (astigmatism and coma), and alignment sensitivity of convergence have been included. The aberration coefficients A.sub.1 -A.sub.18 and B.sub.1 -B.sub.18 can be expressed in integral form. The physical significance of the aberration coefficient becomes clear when the following simplifying assumptions are made; (a) the main deflecting fields of the vertical and horizontal coils are similar, i.e., H.sub.II0 (z).apprxeq.-CH.sub.I0 (z); and (b) their Gaussian deflections are substantially coincident so that X.apprxeq.CY (a scale factor difference C.noteq.1 does not affect the aberration coefficients, which include ratios of the field distribution functions). These are excellent approximations for toroidal yokes, in which the vertical and horizontal windings have the same axial length; and in the case of saddle or saddle-toroid windings, the shorter length of the vertical coils is compensated by their larger inner diameter so the approximations remain valid. The detailed winding distributions of the horizontal and vertical coils are different, and as a result, their nonuniformity functions are dissimilar; H.sub.II2 (z).noteq.-CH.sub.I2 (z).
The simplified aberration coefficients necessary for understanding the invention are then: EQU B.sub.2 +A.sub.3 =(.lambda./4D.sup.2)+2S.sub.II1 +S.sub.I1 ( 5) ##EQU2## EQU A.sub.6 +B.sub.6 =(.lambda./3D)+2(S.sub.II2 +S.sub.I2) (8) EQU A.sub.7 =(3/2)-S.sub.II3 ( 9) EQU B.sub.8 =-1/2+S.sub.I3 ( 10) EQU A.sub.16 =-2S.sub.II4 ( 11) EQU A.sub.18 =2S.sub.II4 -(1/D) (12) EQU B.sub.17 =2S.sub.I4 ( 13) EQU B.sub.18 =2S.sub.I4 -(1/D) (14)
in which D is the distance from the principal plane of Gaussian deflection to the screen, L is the effective length of the deflection yoke, .lambda.=L/D, and S.sub.1, S.sub.2, S.sub.3 and S.sub.4 are defined below.
The terms S.sub.IIi, S.sub.Ii (i=1,2,3,4) are integral expressions containing the functions H.sub.II0, H.sub.II2 and H.sub.I0, H.sub.I2. Thus, for example, North-South pincushion distortion is determined by the coefficient B.sub.2 +A.sub.3 of equations (4) and (5), which includes both: EQU S.sub.II1 =(1/X.sub.s.sup.3).intg.H.sub.II2 X.sup.2 (z-z.sub.s)dz (15)
and EQU S.sub.I1 =(1/Y.sub.s.sup.3).intg.H.sub.I2 Y.sup.2 (z-z.sub.s)dz (16)
Here, X.sub.s and Y.sub.s are the Gaussian deflections on a viewing screen located at z.sub.s with a distance D=(z.sub.s -z.sub.c) from the deflection center z.sub.c of the yoke, and z is distance measured along the longitudinal axis of the yoke. H.sub.II2 and H.sub.I2 are the horizontal and vertical field nonuniformity functions respectively. The integration is formally performed from -.infin. to +.infin. but practically may be considered to begin at a distance approximately one yoke diameter from the entrance of the yoke and to terminate at the screen.
Astigmatism in the horizontal direction is determined by the coefficient A.sub.4, which in turn is partially determined by: EQU S.sub.II2 =(1/X.sub.s.sup.2).intg.H.sub.II2 X(z-z.sub.s).sup.2 dz (17)
Astigmatism in the vertical direction is determined by the coefficient B.sub.5, which in turn is partially determined by: EQU S.sub.I2 =(1/Y.sub.s.sup.2).intg.H.sub.I2 Y(z-z.sub.s).sup.2 dz (18)
Coma is determined by: EQU (horizontal) S.sub.II3 =(1/X.sub.s).intg.H.sub.II2 (z-z.sub.s).sup.3 dz (19) EQU (vertical) S.sub.I3 =(1/Y.sub.s).intg.H.sub.I2 (z-z.sub.s).sup.3 dz (20)
These expressions describe the pincushion, astigmatism and coma distortions considered in the prior art for producing self-converging yokes corrected for N-S pincushion and for coma.
Alignment sensitivity is determined by: EQU (horizontal) S.sub.II4 =(1/X.sub.s).intg.H.sub.II2 (z-z.sub.s).sup.2 dz (21) EQU (vertical) S.sub.I4 =(1/Y.sub.s).intg.H.sub.I2 (z-z.sub.s).sup.2 dz (22)
While all portions of the yoke and its fields affect each of the distortions, the effect of changes in particular regions of the fields may affect particular distortions disproportionately.
This invention is based on the recognition that different portions of the H.sub.2 -functions contribute differently to the sensitivity of convergence to misalignment of the yoke relative to the picture tube of the display system. Three yoke field regions are defined. The entrance region extends from the exit of the electron gun to the vicinity of the entrance plane of the horizontal coils. The exit region extends from the vicinity of the exit plane of the core to the screen. The mid region is bounded by the entrance and exit planes.
The weighting functions appearing in the integrands of S.sub.IIi, S.sub.Ii weight the H.sub.2 -functions as shown in FIG. 1. Under the assumption of similar main deflecting fields, only the horizontal weighting functions need be shown since the weighting functions for the vertical field correspond. In FIG. 1, the abscissa represents axial distance in the display system measured from the deflection center z.sub.c and the ordinate represents the weighting function in arbitrary units. The screen is at a position z.sub.s =10 inches (25.4 cm) from the deflection center. The approximate position of the entrance and exit planes of a deflection yoke are indicated as EN and EX, respectively. The ordinate values are not the same for the different functions.
Equations (15) and (16) indicate that pincushion is determined mainly by the behaviour of the H.sub.2 -functions in the exit region and, to a smaller extent, in the mid region since the magnitudes of the negative weighting functions appearing in these equations, X.sup.2 (z-z.sub.s) and Y.sup.2 (z-z.sub.s), rise very steeply from their low values at the entrance, as illustrated in FIG. 1.
Equations (17) and (18) indicate that the astigmatism required for self-convergence is determined by portions of the H.sub.2 -functions in the mid and exit regions of the yoke since the positive weighting functions X (z-z.sub.s).sup.2 and Y (z-z.sub.s).sup.2 rise rapidly from their values at the entrance.
Equations (19) and (20) indicate that coma is determined mainly by the behaviour of the H.sub.2 -functions in the entrance region and, to a smaller extent, in the mid region since the magnitude of the negative weighting function (z-z.sub.s).sup.3 decreases rapidly from its maximum value at the entrance.
Equations (21) and (22) show that convergence sensitivity to misalignment is determined by the behaviour of the H.sub.2 -functions in the entrance and mid regions and, to a smaller extent, in the exit region, since the positive weighting function (z-z.sub.s).sup.2 decreases less rapidly from its maximum value at the entrance.
Prior-art self-converging yokes for horizontal in-line gun display systems such as the RCA 19V90.degree. toroidal yoke or the Hitachi 17V90.degree. semitoroidal yoke, had field distribution functions as illustrated in FIGS. 2 and 3, respectively. As illustrated in FIGS. 2 and 3, the H.sub.I2 and H.sub.II2 functions are multiplied by a factor of 10 for clarity.
A qualitative discussion of prior art yokes can be based on the weighting functions illustrated in FIG. 1 in conjunction with FIGS. 2 and 3. These yokes had horizontal field nonuniformity functions H.sub.II2 whose positive lobes (pincushion-shaped fields) exhibited excessively large peaks in proximity to the entrance (EN) of the yoke. Such H.sub.II2 -functions produced the negative astigmatism required for convergence of the offset beams along the horizontal axis in an inefficient manner, since pincushion fields located near the entrance of the yoke, where the deflection is still small, must have excessive nonuniformity to achieve self-convergence. This inefficient axial distribution of the H.sub.II2 -functions illustrated in FIGS. 2a and 3a led to sensitivity of convergence to misalignment of the beams in the horizontal fields and contributed to horizontal coma. The aforementioned prior-art yokes had vertical field nonuniformity functions H.sub.I2 with excessively large negative values (barrel-shaped fields) near the entrance of the yoke, and in the case of toroidal vertical coils, as shown in FIGS. 2 and 3, all negative, unbalanced or single-lobe H.sub.I2 -functions. Such H.sub.I2 -functions produced the positive astigmatism required for self-convergence along the vertical axis inefficiently, since the contribution of barrel fields at the entrance of the yoke to astigmatism is small, thus forcing the mid-yoke barrel fields to have excessive nonuniformity for achieving self-convergence. The consequences of this inefficient axial distribution of the H.sub.12 -functions illustrated in FIGS. 2b and 3b were substantial vertical coma, high sensitivity of convergence to misalignment of the beams in the vertical fields, and a significant contribution to NS pincushion that was difficult to correct by the horizontal coils without causing "gullwing" or higher-than-horizontal-frequency distortion of the raster top and bottom.